It is now common that a cryptography technology is used for recording digital contents which consist of video and audio data on a recording medium, and selling the contents or distributing the contents via a network, to prevent a malicious use of the contents, such as tampering and tapping. For example, a content provider encrypts contents before distributing the contents, and a playback apparatus decrypts the content to play back the contents. Such a cryptography function is often hardware-implemented into a playback apparatus to realize real-time playback and to secure tamper resistance.
Meanwhile, there is a demand for changing a cryptosystem to another when the cryptosystem used for encryption is broken or a new cryptosystem is developed.
As a technique to change a hardware-implemented cryptosystem, Patent Document 1 discloses an encryption processing system. In this technique, the encryption processing system includes a programmable processor, and changes a cryptosystem by downloading a program in correspondence with the type of an application and data, and introducing the program into the programmable processor.
In some cases, such a program includes information to be kept secret, such as key data used in the cryptography processing and a programmer's knowledge. Therefore, the program should be acquired in a safe manner.
Patent Document 2 discloses a technique to securely input structure data for structuring a circuit in an FPGA (Field Programmable Gate Array). According to this technique, a decryption circuit is provided in the FPGA, between a reconfigurable part thereof and an input terminal. This decryption circuit acquires and decrypts encrypted structure data, and outputs the structure data to the reconfigurable part of the FPGA.
Patent Document 1: Japanese Laid-open Patent Application Publication No. 10-320191
Patent Document 2: Japanese Laid-open Patent Application Publication No. 2002-50956